The present invention relates to a method for moving a machine element of an automation machine. The invention also relates to a control device for controlling a movement of a machine element of an automation machine.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Machine tools, in particular, are often provided with so-called redundant kinematics. In this case, redundant kinematics are understood as meaning the possibility of moving a machine element, which may be in the form of a tool receiving apparatus or a tool which is clamped in the tool receiving apparatus for example, along a direction with the aid of two separate drive shafts.
FIG. 1 uses a schematically illustrated machine tool 36 to illustrate the principle of redundant kinematics. A carrier 5 can be moved in a direction X with the aid of two linear motors 3 and 4. The guidance of the movement in the X direction is ensured by two columns 1 and 2 in this case. A further column 6 which is used to guide the movement of a second linear motor 7 is fastened to the carrier 5. The linear motor 7 likewise moves in the X direction. The direction of movement of the linear motors 3, 4 and 7 is indicated by depicted arrows 37, 12 and 13. A machine element 8 which is in the form of a tool receiving apparatus within the scope of the exemplary embodiment is fitted to the linear motor 7. A tool 9 is clamped in the tool receiving apparatus.
It goes without saying that the machine tool 36 also has further motors which allow a movement of the machine element 8, for example in the Y and Z directions, but are not illustrated in FIG. 1 for the sake of clarity and since they are irrelevant to understanding the invention.
In order to measure first actual values xc,ist which indicate the position of the column 6 with respect to a stationary machine bed 35 of the machine, the machine 36 has a first measuring device which is not illustrated in FIG. 1 for the sake of clarity. In order to measure second actual values xf,ist which indicate the position of the machine element 8 with respect to the column 6, the machine tool 36 has a second measuring device which is likewise not illustrated in FIG. 1 for the sake of clarity.
If the machine element 8 is intended to be moved to a particular desired position value in the direction X, the problem arises of how the movement required for this purpose is intended to be divided between the two linear motors 3 and 4 and the linear motor 7. Since the linear motor 7 must move only small masses (machine element 8 and tool 9), it is able to carry out dynamic movements (for example movements with high accelerations) in the X direction, whereas the two linear motors 3 and 4 can carry out only relatively sluggish movements on account of the larger masses to be moved by them. It is therefore expedient to divide the movement of the machine element into a first movement component, which is carried out by the two linear motors 3 and 4, and a second movement component which is carried out by the linear motor 7. In this case, the first movement component comprises the movement processes which are not very dynamic, that is to say the low-frequency movement processes, whereas the second movement component comprises the dynamic, that is to say high-frequency, movement processes of the machine element.
FIG. 2 illustrates a commercially available control device 14 which is used to divide a movement of a machine element of an automation machine. The control device 14 has a desired value generating unit 15 which generates desired movement values xsoll for the movement of the machine element 8. In this case, the desired movement values xsoll are usually in the form of desired position values. In this case, the individual desired movement values xsoll are spaced apart equidistantly from one another in terms of time. For example, the desired value generating unit 15 can generate a desired movement value xsoll every millisecond. In this case, the desired value generating unit 15 generates the desired movement values using a predefined parts program which predefines the movements to be carried out by the machine element 8 in the form of instructions. It is noted at this point that the desired value generating unit 15 accordingly generates desired movement values for each of the directions X, Y and Z, FIG. 2 and the subsequent figures illustrating only the desired movement values xsoll, which predefine the movement in the X direction, and the associated drive shafts 20a and 20b which are used to carry out the movement in the X direction. The first actual values xc,ist indicate the first movement component of the machine element 8 by indicating the position of the column 6 with respect to the machine bed 35 within the scope of the exemplary embodiment according to FIG. 1. The second actual values xf,ist indicate the second movement component of the machine element 8 by indicating the position of the machine element 8 with respect to the column 6 within the scope of the exemplary embodiment according to FIG. 1.
The desired movement values xsoll are then supplied to a movement dividing unit 23 which is known from the prior art and uses the desired movement values xsoll to determine first desired values xc,soll for controlling the first movement component and second desired values xf,soll for controlling the second movement component and outputs said first and second desired values. In this case, the first movement component is carried out using the first drive shaft 20a and the second movement component is carried out using the drive shaft 20b. In this case, the first desired values sc,soll are the regulating desired values for regulating the first movement component and the second desired values xf,soll are the regulating desired values for regulating the second movement component of the movement of the machine element 8.
The movement dividing method carried out by the movement dividing unit 23 has been disclosed, for example, in U.S. Pat. No. 6,982,536 B2, which is incorporated herein by reference in its entirety. The movement of the machine element is divided into movement sections and the desired value generating unit accordingly outputs desired movement values xsoll for each movement section in succession. In order to divide the movement, the desired movement values xsoll are filtered using a low-pass filter 24 and the first desired values xc,soll are generated on the output side in this manner. In order to determine the second desired values xf,soll, the first desired values xc,soll are subtracted from the desired movement values xsoll using a subtractor 26 and the second desired values xf,soll for regulating the second drive shaft 20b are generated in this manner. As already stated, this method is prior art. In order to compensate for the temporal delay in the first desired values xc,soll which is caused by the filter 24, U.S. Pat. No. 6,982,536 B2 also discloses the practice of additionally providing a delay unit 25 if necessary in order to likewise temporally delay the desired movement values xsoll in a corresponding manner for adaptation to the delay in the first desired values xc,soll. The delay unit 25 is illustrated using dashed lines in FIG. 2.
In order to regulate the first movement component of the machine element 8, the first desired values xc,soll are supplied, as regulating desired values for regulating the first movement component of the machine element 8, to a first regulating means 16a. The first regulating means 16a drives a first power converter 17a, which is illustrated by an arrow 18a in FIG. 2, in accordance with the first desired values xc,soll and the first actual values xf,ist which are measured using a first measuring device 10 and indicate the position of the machine element 8 with respect to the machine bed 35 within the scope of the exemplary embodiment according to FIG. 1. The first power converter 17a accordingly drives the two linear motors 3 and 4, which is illustrated by an arrow 19a, the linear motors 3 and 4 moving a load 19. In this case, the load 19 comprises all elements which are moved by the linear motors 3 and 4 in the direction X. The first regulating means 16a, the first power converter 17a, the linear motors 3 and 4, the load 19 and the measuring device 10 form a first drive shaft 20a which is used to carry out the first movement component of the machine element 8. The first actual variable xc,ist indicates the first movement component of the machine element 8 by indicating the position of the column 6 with respect to the machine bed 35 within the scope of the exemplary embodiment according to FIG. 1.
In order to regulate the second movement component of the machine element 8, the second desired values xf,soll are supplied, as regulating desired values for regulating the second movement component of the machine element 8, to a second regulating means 16b. The second actual values xf,ist which are measured using a second measuring device 11 and indicate the position of the machine element 8 with respect to the column 6 within the scope of the exemplary embodiment according to FIG. 1 are also supplied to the second regulating unit 16b as regulating actual values. The second actual values xf,ist indicate the second movement component of the machine element 8 by indicating the position of the machine element 8 with respect to the column 6 within the scope of the exemplary embodiment according to FIG. 1.
The second regulating means 16b drives a second power converter 17b, which is illustrated by an arrow 18b in FIG. 2, in accordance with the second desired values xf,soll and the second actual values xf,ist. The second power converter 17b accordingly drives the linear motor 7, which is illustrated by an arrow 19b, the linear motor 7 moving a load 21. In this case, the load 21 comprises all elements which are moved by the linear motor 7 in the direction X. The second regulating means 16b, the second power converter 17b, the linear motor 7, the load 21 and the measuring device 11 form a second drive shaft 20b which is used to carry out the second movement component of the machine element 8.
It is noted at this point that the desired value generating unit 15 likewise generates corresponding desired values for controlling the movement of the drive shafts which are used to move the machine element in the Y and Z directions. These desired values and the drive shafts which are used to move the machine element in the Y and Z directions are not illustrated in FIG. 2 and the subsequent figures for the sake of clarity and since they are irrelevant to understanding the invention.
In this case, the desired value generating unit 15 generates the desired values xsoll in such a manner that the desired value generating unit 15 complies with predefined control restrictions to which the movement of the machine element and the movement of the drive shafts involved are subject. The machine element 8 can thus be moved in the X direction only at a maximum speed Vx, with a maximum acceleration Ax and with a maximum jerk Jx since, for example, the force produced by the relevant drive shaft for the purpose of accelerating the machine element in the X direction allows only the maximum acceleration Ax in the X direction, for example. The desired movement values xsoll for the movement of the machine element in the X direction are therefore always generated by the desired value generating unit 15 in such a manner that:|x′soll|≦Vx |x″soll|≦Ax |x′″soll|≦Jx where x′soll is the derivative of xsoll with respect to time t and x″soll is the second derivative of xsoll with respect to time t and x″′soll is the third derivative of xsoll with respect to time t.
If there is only one drive shaft for each direction of movement, no problems arise when moving the machine element. If, however, the machine has redundant kinematics, that is to say two drive shafts are available for carrying out the movement in the X direction for example, it is often the case with commercially available machines that the first desired values xc,soll determined by the movement dividing unit 23 and/or the second desired values xf,soll determined by the movement dividing unit 23 exceed the movement restrictions of the first drive shaft 20a and/or second drive shaft 20b. This results in the machine element 8 not being moved in the manner predefined by the first desired value xc,soll and the second desired value xf,soll, which results in machining errors, which may be in the form of contour errors for example, and, in the worst case scenario, may result in the machine being switched off if the corresponding drive shaft is overloaded to an excessive extent.
It would therefore be desirable and advantageous to provide an improved method to obviate prior art shortcomings and to avoid overloading of the drive shafts in an automation machine having redundant kinematics.